Honeycomb core panel construction which is made from aluminum or one of its alloys is highly susceptible to corrosion caused by atmospheric moisture. Honeycomb core panel construction utilizes two outer "skin" layers that sandwich a honeycomb-shaped core, and is used extensively in the fabrication of fins, wings and other structure in virtually all aircraft, including high-performance tactical aircraft such as the EF-111A and the F-14. Atmospheric moisture often seeps into and becomes trapped between the outer skin layers of the honeycomb core panel construction. Over time, the water seepage corrodes the honeycomb-shaped core, making the aircraft structurally unsound and, therefore, unsafe for flight. The eventual result is that the entire aircraft control surface made from the deteriorated honeycomb core panel construction must be either rebuilt or replaced at a great cost.
The formation of protective coatings on metal sheets for increasing corrosion resistance is known. For example, U.S. Pat. No. 4,894,127 disclosed an anodizing process which is used to form a 1 to 3 micro thick coating of aluminum oxide onto the surface of an aluminum plate. Another process for increasing corrosion resistance involves cladding films of pure aluminum 0.002-0.005 inches thick onto the surface of sheet metal. Still another technique for increasing corrosion resistance, which is disclosed in U.S. Pat. No. 5,007,225, involves the application of paints, foams and other anti-corrosion materials to the inner and outer surfaces of metal sandwich panel structures, which are used primarily in the fabrication of ship hulls and bulkheads.
The use of surface coatings to increase the corrosion resistance of honeycomb core panel construction is undesirable for two reasons. First, the honeycomb-shaped core is left vulnerable to corrosion in those areas where the surface coatings fail to cover. Corrosion in these exposed areas eventually causes the deterioration of the entire panel, which results in the repair or replacement of the aircraft control surface at a significant cost. Second, surface coatings applied to the honeycomb-shaped core will cause a substantial increase in the weight of an aircraft, which compromises aircraft flight efficiency and performance.
An alternative corrosion prevention technique, implantation of ions into the surface of a metal, has been used for increasing wear and corrosion resistance in metals in contexts unrelated to honeycomb core panel construction. For example, U.S. Pat. No. 4,743,308 discloses a method wherein ion implantation is used to change the surface characteristics of titanium parts which are used in the construction of human body joint implants. The penetration of ions into the titanium surface creates an alloy which improves the wear and corrosion resistance of the metal without the drawbacks associated with the use of protective coatings. U.S. Pat. No. 4,693,760 specifically discloses implanting molybdenum ions into the surface of a metal workpiece. Although extensively used in the above-stated applications, however, no known ion implantation process exists for increasing the corrosion resistance of honeycomb core panel construction.
The lack of an effective technique for preventing the corrosion of aluminum honeycomb core panel construction has led aircraft designers to substitute heavier, less-corrosive metals or alternative standard construction techniques. Heavier substitute metals substantially increase aircraft manufacturing costs and significantly compromise aircraft flight efficiency and performance. Even the lighter substitute metals, such as titanium, produce undesirable effects in the form of increased aircraft weight and cost. Non-corrosive, non-metallic materials may be substituted. However, such materials are also undesirable because, in addition to increasing cost, they possess a low strength-to-weight ratio.
Because aluminum has the best weight-to-strength ratio of commonly used metal, it is imperative to retain the ability to use aluminum honeycomb core panel construction in the design of aircraft control surfaces. Therefore, a need exists to provide an effective method for increasing the corrosion resistance of aluminum honeycomb core panel construction.